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February 2002
Product Description
11
1.2 Transmitter
The transmitter power stage is supplied with power via two separate supply lines
VSP and GNDP. Because of the high current requirements for the transmitter
power stage, these supply lines are separated from the logic section supply lines
and have two pins per line.
The ground pins for the logic section and the transmitter are not connected
internally, in order to avoid possible problems with a high resistivity of GNDP pins
and in order to increase flexibility when using long supply lines. Pins GND and
GNDP must be connected to each other externally. For more details, refer to
Chapter 3.2, Power Supply Connection.
The transmitter power stage is internally connected to the supply lines GNDP and
VSP via a common-mode choke coil in order to reduce electromagnetic
interference (EMI) on the supply lines.
The regulated transmitter power stage supply may vary b7V and +24V.
The supply lines VSP and VSL should be connected together when the supply
voltage is +7 V or more. For details refer to Section 2, Specifications.
Note:
The RA-RFM has an in-built temperature protection circuit which
sharply limits the transmitter power stage output if an over-current
situation or an over-temperature environment causes the temperature
to exceed the allowed limits. After the device is switched off and has
time to recover (when the temperature drops again or the over-current
situation is otherwise rectified) the unit reverts to normal operation
when it is switched on again. Such an occurrence is an indication that
the RA-RFM is not being operated within specification.
The transmit frequency (134.2 kHz) from the oscillator is fed to the Pulse Width
Modulator (PWM). By changing the value of a resistor, the PWM can set the pulse
width ratio between 0% and 50%. For an example of two different oscillator signal
pulse widths see Figure 3. Decreasing the 134.2 kHz frequency pulse width ratio
decreases the generated transmit (charge-up) field strength.
It is therefore possible to adjust the generated field strength by selecting different
pulse width ratios. For more information about setting the field strength, refer to
Appendix 1, Field Strength Adjustment.
Figure 3: Pulse Width Examples
Pulse width of 50%
Pulse width of 12.5%
Summary of Contents for 2000 series
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